Bacteria Beware!

The Life and Times of Bdellovibrio

by Laurel Crosby

A mild ocean breeze plays over the water surface,
dispelling any notion that danger lurks in the murky depths.
However, a gruesome event is about to occur as a silent
attacker speeds forth toward an unsuspecting victim. In a
furious collision, the savage meets its target and whittles
its way into the body of the innocent prey. Once inside, the
transformation begins - the predator ceases its frenzy and
prepares to multiply. The host is reduced to a protective
cocoon, supplying food and shelter for the growing parasite.
Within hours, the nourishment is drained and the ghost-like
shell of the host bursts open to release a new generation of
deadly predators. And all the while, the waters remain
still...

While the above description may sound like the season's next
cinema thriller, it actually describes the life cycle of the
bacterium, Bdellovibrio. The Bdellovibrio (which
literally means "curved leech") make a living by attacking and
devouring other bacteria, and are found in diverse environments such
as marine and fresh waters, sewage, and soil. Bacteria of these type
are characterized by two distinct stages in their life cycle, a
predatory "attack" phase, and a parasitic "growth" phase.

The attack phase cell has a curved rod shape of approximately 1.4
micrometers (or microns) in length, and has a single hair-like
projection called a flagellum. This bacterial flagellum rotates like
a corkscrew to propel the bacterium at a rate of 100 microns per
second. Considering the size of the cell, this corresponds to an
incredible 70 body lengths per second! These highly motile attack
phase cells have no sense of direction; instead, the flagellum
propels the cell in whatever direction it happens to be pointing.
Success in finding prey, therefore, is limited to "bumping" into a
suitable prey cell that just happens to be in the right place at the
wrong time. When a prey cell is encountered, the Bdellovibrio
continues to rotate and bore its way into the prey. Once inside, the
attacker loses its flagellum and prepares for the multiplication
process.

The Bdellovibrio growth phase is the period when new cells
are created and requires the parasitism of a suitable host cell. The
attacker cells are not especially particular about the prey, except
that it must be of the Gram negative type (i.e. having a thin cell
wall and characteristic outer membrane.) When the Bdellovibrio
enters, the host bacterium dies and bloats into a spherical shape
called a bdelloplast. Essentially, the host cell loses its
structural framework because it is being eaten from the inside out.
The growing Bdellovibrio is now considered a parasite and
continues to elongate into a filament. When the nutrients (proteins,
lipids, structural polymers, RNA, and DNA, etc.) are exhausted from
the host body, the filament partitions into the smaller attack phase
cells and are released into the environment.

The unique lifestyle of the Bdellovibrio is of keen
interest to scientists. We are curious as to why these organisms
require a bacterial host in order to multiply. Could the host cell be
supplying an essential nutrient? A vitamin? Perhaps a protein or two?
Studies show that normal Bdellovibrio can multiply without
living host cells under certain conditions. If the soluble components
are extracted from host cells and fed into a beaker containing only
Bdellovibrio, the Bdellovibrio are able to grow and
multiply. This suggests that there is a factor which
Bdellovibrio needs in order to survive - and only prey
bacteria can provide! We are still searching for the factors that
control the host-parasite relationship and we are continuing to make
new discoveries. Meanwhile, Bdellovibrio remains a fierce
predator in the natural environment.